Electrokinetic behavior of spherical colloidal particles of cadmium sulfide

Abstract

The electrophoretic behavior of monodisperse, spherical cadmium sulfide particles is analyzed in this work with the aim of contributing new data on the electrical nature of the surface of this material. The electrophoretic mobility, μ e, of CdS particles is studied as a function of both pH and concentration of several electrolytes. As in many metal sulfides, the surface characteristics are very sensitive to its degree of oxidation, as demonstrated by the slight increase in isoelectric point (iep) of samples obtained with longer synthesis times. The iep of CdS is found to be between pH 1 and 1.5, this value being unaltered by the addition of NaCl at different concentrations. The effect of the lattice ions (Cd 2+ and S 2−, or rather HS −) on the mobility is very significant: HS − anions adsorb on the particles, increasing the negative values of μ e whereas the behavior in the presence of Cd 2+ salts suggests surface precipitation of Cd(OH) 2. The cations Ag +, Cu 2+, Mn 2+ and La 3+ can be considered as activating species for the cadmium sulfide/solution interface: their hydrolysis products adsorb onto the CdS particles and provoke up to three pH values of inversion of the sign of μ e. The effect is most important for Cu 2+, Ag + and La 3+ cations, for which μ e reversals can be achieved for initial concentrations as low as 10 −4 and 5 × 10 −5 M, respectively. If MnCl m2 solutions are used, their concentration must be ⩾ 10 −3 M to observe the same phenomenon.